(a) Field of the Invention
This invention relates to optically addressed spatial light modulators, which are electro-optical devices whereby a writing light controls the modulation of a reading light according to the image pattern existing in the writing light beam. Such devices have many uses such as, for example, allowing an image illuminated with light of weak intensity to be projected with light of strong intensity, converting an image in light of one wavelength to an image in light of another wavelength, or converting an image in incoherent light to an image in coherent light.
(b) Description of the Prior Art
Examples of electro-optic devices which allow the modulation of light to be controlled by light, and which specifically make use of spatial variation or the image content of the controlling light, are generally known in the prior art. The type most similar to that of the present invention is frequently called a liquid crystal light valve. These devices comprise a liquid crystal light modulating layer in contact with a photoresponsive layer, typically a photoconductor Changes in the light illuminating the photoresponsive layer (write light) cause corresponding changes in the electric fields applied to the liquid crystal layer, further causing corresponding changes in light passing through the liquid crystal (read light).
Typical liquid crystal light valves, such as the one disclosed in U.S. Pat. No. 3,824,002, use nematic liquid crystals as the light modulator. Nematic liquid crystals have several advantages for use in these devices in that they exhibit large modulations for relatively small changes in applied voltage and their operation consumes very little electrical power. However, nematic liquid crystals respond only to changes in the magnitude but not in the polarity of applied electric fields. Therefore, although they can be switched in one direction increasingly fast by increasingly large applied fields, switching them in the opposite direction can only be accomplished by removing the electric field. The switching then proceeds under the drive of comparatively small surface elastic forces. The combination of the small surface elastic forces with the nematic liquid crystal's viscosity limits the response speed of all such devices to the millisecond range, as is well known in the art. Also, nematic liquid crystal layers used are typically 5-20 .mu.m thick, which limits the spatial resolution as well, for it is not possible to switch regions of the liquid crystal that are much smaller in extent than the thickness of the film.
The liquid crystal light valves of the prior art also typically use photo-conductive materials such as the CdS layer taught by U.S. Pat. No. 3,824,002 for their photoresponsive layers. While such photoconductors are usually very sensitive, producing changes in the charge of many electrons for every absorbed photon, they are also usually slow, taking several milliseconds to respond to changes in their illumination level. Furthermore, many of the easiest to use photoconductive materials exhibit significant responses only to light of relatively short wavelengths as found in the blue end of the visible spectrum.
Alternately, other disclosures in the prior art such as that by Efron et al., "The silicon liquid-crystal light valve," Journal of Applied Physics, volume 57, p. 1356 (1985), that by Armitage et al., "Gallium arsenide photoaddressed liquid-crystal spatial light modulator," Advances in Optical Information Processing III, Dennis R. Pape, editor, Proceedings SPIE, volume 936, pp. 56-67 (1988) and those of U.S. Pat. Nos. 4,191,452, 4,239,348, 4,619,501, and 4,655,554 teach the use of crystalline silicon or gallium arsenide as the photoresponsive layer. These materials have favorable response times and are sensitive to longer wavelengths, but they have their own disadvantages. For example, they must be fabricated in single crystal form, which is difficult and expensive. Also, since a wafer of these semiconductors is obtained by sawing from a large single crystal, it is not possible to obtain thin, optically flat layers as desired for the liquid crystal light valve. Moreover, the only way to obtain reasonable spatial resolution with such a thick layer is to add further patterns of doping and metal deposition. Such additional steps are difficult and produce a light valve whose resolution is limited by the resolution of the patterning steps.
In the operation of any liquid crystal light valve, care must be taken that the reading or projection light does not also write the device. Devices of the prior art have typically used one of two methods to accomplish this isolation. The first method involves placing a dielectric mirror of high reflectivity at the interface between the photoresponsive layer and the liquid crystal, as taught in U.S. Pat. No. 3,824,002. By reflecting or blocking substantially all of the light incident from the liquid crystal side of the device from reaching the photoresponsive layer, the reading and writing functions can be separated. However, such a dielectric mirror, in order to be efficient, must be at least as thick as several wavelengths of the light it is to reflect, and thus it comprises many layers. Also, no matter how fine a pattern of electric charge can be generated at the surface of the photoresponsive layer, the electric fields resulting across the liquid crystal layer can change substantially over distances comparable to the thickness of the mirror layer, thereby limiting the resolution of the device. The second method involves placing a metal mirror at the same interface. However, since metals conduct electricity, the mirror must be patterned if the device is to be able to respond to images. Methods of fabricating such a device are taught in U.S. Pat. No. 4,538,884. Again, the maximum resolution achievable is equal to the resolution with which the metal mirror can be patterned.
Most of the devices of the prior art are constituted such that continuing modulation of the read light requires continuing illumination by the writing light. Such a requirement prevents the desirable function of integration, whereby further increments of exposure to the write light produce more or less irreversible increments in the modulation of the read light. Photographic film provides such an integration function whose advantages are readily apparent to those skilled in the art. However, film requires the separate and time consuming extra step of development.
Both fast liquid crystal light modulators and fast photoresponsive layers are separately known in the prior art. For example, U.S. Pat. Nos. 4,367,924 and 4,563,059 teach the use of chiral tilted smectic liquid crystals which are ferroelectric. These ferroelectric liquid crystals (hereafter "FLCs") have the property, unlike nematics, of being sensitive to the sign or polarity of an applied electric field. This allows them to have two states, into either of which they can be driven by externally applied electric fields, thereby obviating the need to rely on the much weaker internal surface elastic forces. FLCs can be switched in microseconds, which is quite fast compared to the nematics' millisecond switching time. However, FLCs cannot simply be substituted for nematics in an optically addressed spatial light modulator. As pointed out by Armitage et al in "Photoaddressed ferroelectric liquid crystal devices," Optical Society of America Annual Meeting, 1988, Technical Digest Series, Volume 11 (Optical Society of America, Washington, D.C., 1988), p. 118, photoaddressing structures that are effective in addressing nematic liquid crystal cells often prove ineffective for addressing ferroelectric liquid crystal cells.
Photoresponsive layers of hydrogenated amorphous silicon (hereafter a-Si:H) have been investigated extensively over the past two decades, with much work directed toward their use in photovoltaic solar cells. As is known in the art, a-Si:H photoresponsive layers are used in vidicons and photocopy drums. The material may be used either with ohmic contacts, as a photoconductor, or with rectifying layers, as a photodiode, as in the solar cells. The photodiodes are known to have microsecond response times to changes in illumination. Use of a-Si:H combined with nematic liquid crystals in optically addressed spatial light modulators is taught in U.S. Pat. No. 4,538,884 to Masaki and U.S. Pat .No. 4,693,561 to Davis and in Ashley et al, "Amorphous silicon photoconductor in a liquid crystal spatial light modulator," Applied Optics, vol. 26, pp. 241-246 (1987) and "Liquid crystal spatial light modulator with a transmissive amorphous silicon photoconductor," Applied Optics, vol. 27, pp. 1797-1802 (1988).
Finally, Moddel et al. disclose in "Photoaddressing of High Speed Liquid Crystal Spatial Light Modulators," Optical and Digital Pattern Recognition, Hua-Kuang Liu and Paul Schenker, editors, Proceedings SPIE, vol. 754, pp. 207-213 (1986); "Design and Performance of High-speed Optically-addressed Spatial Light Modulators," Advances in Optical Information Processing III, Dennis R. Pape, editor, Proceedings SPIE, vol. 936, pp. 48-55 (1988); and "Optical Addressing of High-speed Spatial Light Modulators with Hydrogenated Amorphous Silicon," Materials Research Society Proceedings, vol. 118, pp. 405-410 (1988) and Williams et al. disclose in "An Amorphous Silicon/Chiral Smectic Spatial Light Modulator," Journal of Physics D: Applied Physics, vol. 21, pp. 156-159 (1988), the use of liquid crystal light valves having photoresponsive layers made of a-Si:H and having light modulating layers made of FLCs. Such devices appear to have the potential for overcoming many of the short-comings of the prior art devices recited above. Namely, both the photoresponsive layer and the light modulating layer are thin so as to support a high spatial resolution and fast switching, thereby allowing response times in the microsecond range.
However, those skilled in the art will appreciate that the devices disclosed in the above publications have their own difficulties. Specifically, the a-Si:H layer has a large capacitance, which makes it difficult to take such a device from an erased state to a light sensitive state. The necessary changes in the applied voltages feed through the layer's capacitance and write the liquid crystal layer even in the absence of writing illumination. Moddel et al teach that this problem may be solved by reducing the amplitude of the driving voltage waveform. Unfortunately, this also increases the optically addressed spatial light modulator's response time since the FLC responds more slowly to a smaller applied voltage.